No reduction using sublimination of cyanuric acid

ABSTRACT

A method of reducing the NO content of a gas stream comprises contacting the gas stream with an amount of HNCO at a temperature effective for heat-induced decomposition of cyanuric acid, said amount and temperature being effective for the resultant lowering of the NO content of the gas stream, said cyanuric acid being particulate and having a particle size of less than 90 μm.

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC04-76DP-00789 between the U.S. Department of Energy and AT&TTechnologies, Inc.

This application is a continuation application of application Ser. No.08/004,034, filed Jan. 15, 1993, now abandoned which is a continuationof application Ser. No. 07/491,996, filed Mar. 12, 1990, now U.S. Pat.No. 5,180,565, which is a divisional of application Ser. No. 07/154,247of Jan. 5, 1988, now U.S. Pat. No. 4,908,193, which is the NationalPhase of PCT/US87/01029 of May 5, 1987, which is a C-I-P of applicationSer. No. 07/859,951 of May 5, 1986, now U.S. Pat. No. 4,731,231.

BACKGROUND OF THE INVENTION

This invention relates to a new method and device for removing NO_(x)from gaseous material, e.g., from exhaust gas streams.

The recent emphasis on ecological and environmental concerns, especiallyair pollution, acid rain, photochemical smog, etc., has engendered awide variety of proposed methods for removing NO_(x), especially NO fromgas streams.

Certain proposed techniques involve a great deal of capital outlay andrequire major consumption of additives, scrubbers, etc. For example,U.S. Pat. No. 3,894,141 proposes a reaction with a liquid hydrocarbon;U.S. Pat. No. 4,405,587 proposes very high temperature burning with ahydrocarbon; U.S. Pat. No. 4,448,899 proposes reaction with an ironchelate; and U.S. Pat. No. 3,262,751 reacts NO with a conjugateddiolefin. Other methods utilize reactions with nitriles (U.S. Pat. No.4,080,425), organic N-compounds (e.g., amines or amides) (DE 33 24 668)or pyridine (J57190638). Application of these reactions imposes organicpollutant disposal problems along with the attendant problems oftoxicity and malodorous environments. In addition, they require thepresence of oxygen and are relatively expensive.

Other systems are based on urea reactions. For example, U.S. Pat. No.4,119,702 uses a combination of urea and an oxidizing agent whichdecomposes it, e.g., ozone, nitric acid, inter alia; U.S. Pat. No.4,325,924 utilizes urea in a high temperature reducing atmosphere; andU.S. Pat. No. 3,900,554 (the thermodenox system) utilizes a combinationof ammonia and oxygen to react with nitric oxide. All of these methodsmust deal with the problem of the odor of ammonia and its disposal. Allrequire oxygen or other oxidizing agents. These methods also suffer fromthe drawback of requiring controlled environments which make themdifficult to use in mobile vehicles or smaller stationary devices.

Japanese Publication J55051-420 does not relate to the removal of nitricoxide from gaseous systems, at least as reported in Derwent Abstract38871C/22. It utilizes halocyanuric acid to remove malodorous fumes,e.g., mercaptans, sulfides, disulfides, ammonia or amines from gases bycontact therewith followed by contact with activated carbon.Temperatures are reported as less than 80° C.; classical acid/baseinteractions appear to be involved (not pyrolysis decomposition productsof the halocyanuric acid).

Back et al, Can. J. Chem. 46, 531 (1968), discusses the effect of NO onthe photolysis of HNCO, the decomposition product of cyanuric acid. Anincrease of nitrogen concentration in the presence of large amounts ofnitric oxide (torr levels) was observed utilizing a medium pressuremercury lamp for photolysis of HNCO. High temperature reactions wereneither addressed nor involved; similarly, the effect, if any, of PINCOunder any conditions on low amounts of NO (e.g., in the <torr to ppmrange) was also not addressed. In fact, the increased concentration ofnitrogen was associated by the authors with high NO levels. Theirtheorized reactions explaining the results would be important only athigh NO levels.

Furthermore, use of cyanuric acid as a source of isocyanic acid (HNCO)for purposes of studying various properties of the latter or itssubsequent degradation products is also known. See, e.g., Okabe, J.Chem. Phys., 53, 3507 (1970) and Perry, J. Chem. Phys., 82, 5485 (1985).J.P. 53-28771 discloses the addition of relatively large particles(0.1-10 mm, preferably 0.5-5.0 mm) of cyanuric acid at temperaturesgenerically disclosed as 600°-1500° C., but preferably at hightemperatures of 1200°-1300° C., for removal of NO_(x) from exhaust gas.The theory of operation disclosed in this publication appears to involvea reaction occurring on the surface of the particle which leads to therequirements of the high particle size and high temperature. It isexplicitly stated in the publication that, "If the diameter of thegranule is too small, the efficiency goes down." There is no suggestionin this publication that the active species effecting the treatment ofthe exhaust gas is itself gaseous and certainly no suggestion that thegaseous species is HNCO. As a result, the conditions disclosed in thisreference lead away from those which are most applicable to a reactionof NO_(x) with gaseous HNCO. Consequently, the process of this referenceis believed not to have been used on a technical scale.

As a result, there continues to be a need for a simple, relativelyinexpensive, non-polluting, non-toxic, non-malodorous and regenerablesystem, method and device for removing nitric oxide from gas streams.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide such a system,method and device.

It is another object of this invention to provide such a method, systemand device which is applicable to small stationary devices, mobilevehicles, as well as to larger applications, including smokestacks fromplants, furnaces, manufacturing factories, kilns, vehicles, andessentially any other source of exhaust gas containing NO, particularlyindustrial gases.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

These objects have been attained by this invention by providing a methodof reducing the NO content of a gas stream comprising contacting the gasstream with HNCO at a temperature effective for heat induceddecomposition of HNCO and for resultant lowering of the NO content ofthe gas stream. It is preferred that the HNCO be generated bysublimation of cyanuric acid.

In another aspect, these objects have been achieved by providing adevice useful for reducing the NO content of a gas stream comprising:

means for storing a compound which upon sublimation generates HNCO;

means for subliming said compound in operation;

means for contacting said NO-containing gas stream with said generatedHNCO; and

means for raising the temperature of said gas contacted with HNCO to alevel effective for heat induced decomposition of HNCO and resultantLowering of the NO content of the gas stream.

In yet another aspect, these objects have been achieved by providing ina conduit means for an effluent gas stream containing NO, theimprovement wherein the conduit means further comprises device means forlowering the NO content of said gas, said device means comprising:

compartment means for storing a compound which upon sublimationgenerates HNCO;

means for heating said compound to a temperature at which it sublimes;

means for contacting said NO-containing gas stream with said generatedHNCO; and

means for raising the temperature of said HNCO-contacted gas stream to alevel effective for heat induced decomposition of HNCO and resultantlowering of the NO content of the gas stream;

and by providing a method of reducing the NO content of a gas streamcomprising contacting the gas stream with an amount of HNCO at atemperature effective for heat induced decomposition of HNCO, saidamount and temperature being effective for resultant lowering of the NOcontent of the gas stream, wherein said contacting is carried out in thepresence of a surface which is effective as a catalyst for at least oneof the reactions between heat induced decomposition products of HNCO andother components in said gas stream, which reactions lead to lowering ofthe NO content of the gas stream, or is effective as a catalyst forgenerating free radicals in the thermal decomposition of HNCO,

and a method of reducing the NO content of a gas stream comprisingcontacting the gas stream with an amount of HNCO at a temperatureeffective for heat induced decomposition of HNCO, said amount andtemperature being effective for resultant lowering of the NO content ofthe gas stream and wherein said contacting is conducted under conditionswhereby the reactions lowering said NO_(x) content occur substantiallysolely in the gas phase between NO_(x) and the products of saiddecomposition of HNCO and not on the surfaces of any solid particlesadded to the gas stream.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and attendant advantages of the presentinvention will be more fully appreciated as the same becomes betterunderstood when considered in connection with the accompanying drawings,in which like reference characters designate the same or similar partsthroughout the several views, and wherein:

FIG. 1 schematically illustrates-one possible configuration for carryingout the method of this invention and for configuring the device and/orimproved conduit of this invention.

DETAILED DISCUSSION

This invention provides many significant advantages over othertheoretical and/or commercially available NO reducers. It is genericallyapplicable to all industrial gas effluent streams, e.g., those mentionedin the references discussed above. It is very simple, inexpensive andportable. It does not require the use of catalysts and/or co-agents. Inaddition, when the preferred source of HNCO (cyanuric acid) is spentduring operation, it can be simply and inexpensively replaced. Itprovides heretofore unachievable convenience and efficiency in reducingNO. Its non-toxicity is another major advantage as its readyavailability and low cost.

As opposed to many of the other systems now available, that of thisinvention imposes minimal changes in otherwise preferred operatingconditions for the engine, plant, factory, etc., which generates theeffluent gas stream being purified. For example, as opposed to presentlyutilized catalytic converters, this invention does not impose arequirement that a vehicular engine be run rich with result-antundesirable lower compression ratios. In addition, the requirement foruse of unleaded gas in order to avoid catalyst poisoning also does notapply. Overall, the efficacy of the system of this invention in loweringNO contents is extremely high.

Within the broadest scope of this invention, any source and/or means ofgenerating HNCO and admixing it with the effluent stream can be used.For a variety of reasons including those discussed above, In thepreferred embodiment, sublimation of cyanuric acid will be utilized:##STR1##

Isocyanuric acid is a tautomer of cyanuric acid. For purposes of thisinvention, the two are equivalent. The sublimation of cyanuric acid inaccordance with the following equation, ##STR2## can be conducted at anytemperature effective to cause a volatilization of sufficient HNCO forthe desired purpose. In general, temperatures greater than 300° C. willbe utilized since sublimation rates at lower temperatures are generallytoo low. Preferably, temperatures greater than 320° C. will be used,especially greater than 350° C. There is no preferred upper limit ontemperature; but generally a temperature less than about 1200° C. willbe employed. The precise temperature for a given application can beroutinely selected, perhaps with a few orientation experiments, inconjunction with considerations of the volume to be filled, the flowrate of the gas, the resultant residence time of the admixture of HNCOand NO in the effluent gas stream, the surface area of the HNCO sourcewhich is being sublimed and the sublimation rate which ensues in a givensystem upon selection of the given temperature. For example, for 50 g ofa cyanuric acid sample having a surface area of about 20 cm², thesublimation rate achieved at a temperature of 450° C. is sufficient toreduce the NO level from a 50 l/m gas stream from 1000 ppm toessentially 0 ppm.

While cyanuric acid itself is the preferred source of HNCO, othersublimable solids can also be used for its generation. These includeother compounds which are typical impurities in samples of cyanuricacid, including ammelide and ammeline ##STR3## In general, cyanuric acidwherein the OH groups are replaced by 1-3 NH₂, alkyl, NH-alkyl orN-alkyl₂ groups, are utilizable. Such alkyl groups typically will have1-4 carbon atoms.

Also utilizable are oligomers of HNCO which are linear rather thancyclic as in cyanuric acid. For example, cyamelide is particularlynoteworthy. Also utilizable are the known halocyanuric acids such as themono-, di- or tri-chloro, bromo, fluoro or iodo acids or other variousmixed-halo substituted acids.

Any means or technique which results in admixture of HNCO with theNO-containing gas is included within the scope of this invention. Forexample, if the effluent gas stream itself is at a sufficiently elevatedtemperature, it can be directly passed over a solid sample of the HNCOsource to effect sublimation and instantaneous admixture. It is alsopossible to incorporate the solid HNCO source into a solvent therefor,most preferably hot water, and conventionally spray or inject thesolution into the effluent gas stream. Of course, it is also possible touse conventional heating means (e.g., conductive, inductive, etc.) toheat the sublimable source of HNCO and then to conventionally conductthe resultant HNCO gas into admixture with the effluent stream. Steaminjection preceded by passage of the steam over, through, etc., the HNCOsource such as cyanuric acid can, of course, also be utilized.

It is also possible to indirectly admix the HNCO with the effluent gasstream. For example, if the HNCO is injected into the combustion chamberwhich produces the effluent gas stream or if the sublimable source suchas cyanuric acid is so injected, the HNCO will be incorporated into theeffluent gas stream at its point of generation. As long as the necessaryreaction conditions are maintained for subsequent interaction of theHNCO with the NO in the gas stream, the NO reduction method of thisinvention will be accomplished. The latter option pertains to any systemwhich generates an NO-containing stream, including vehicular engines(wherein the injection of cyanuric acid or HNCO can be accomplished viathe conventional valves), furnaces, plants, etc. Alternatively, theadmixture can be effected directly either downstream from the point ofgeneration of the effluent gas or directly near or at this point, e.g.,right at the head of the vehicular engine where the heat generated bythe latter can be utilized, not only for sublimation of the solid sourceof HNCO, but also for effecting the NO reducing reactions based on thepresence of HNCO.

The NO content of the effluent streams into which the HNCO has beenadmixed will be lowered as long as the temperature of the effluentstream is raised to a level at which HNCO thermally decomposes intoproducts which result in lowering of the NO content. However, there willbe an upper limit beyond which the nature of the predominant reactionsensuing from the decomposition of HNCO will change in such a fashionthat the desired reduction in NO_(x) will not occur. At elevatedtemperatures, greater than 1200° C., the presence of oxygen will makethe production of nitric oxide un-acceptable. Thus, the temperature atwhich the decrease in the effectiveness of this invention will occur ison the order of 1200° C. and higher.

More generally, the preferred upper limit on the temperature will beless than 1200° C. (e.g., less than 1190°) in dependence on the usualfactors, including diameter of the effluent stream, its velocity, theparticle size of the added HNCO-generating agent where applicable, thegas or particle injection technique and configuration, the residencetime of the reaction, e.g., the length available for the reaction, theinvolved concentrations and amounts of NO_(x) and agent of thisinvention, etc. Thus, in dependence on such factors, the upper limit canencompass a wide variety of values on the order of the mentioned "lessthan 1200° C.", e.g., less than values such as 1195, 1190, 1175, 1150,1125, 1100, 1075, 1050, 1025, 1000, 975, 950, etc. Typically, the upperlimit will fall in the range of 1100°-1200° C. or higher. Thisrelatively lower temperature regime is one aspect of the presentinvention which clearly distinguishes it from the treatment of JP54-28771.

The preferred temperature of operation for a given system will againvary with the usual considerations such as those mentioned above.Typically, the higher available temperatures will be preferred becauseof the more favorable reaction kinetics associated therewith. Thus,reactions will often preferably be conducted in the range of 700°-1100°C., more preferably 900°-1050°, especially 850°-1000° C. One of themajor advantages of this invention is that the reaction, once initiated,will continue to occur to a substantial degree at significantly lowertemperatures, i.e., from the sublimation point of the solid agent suchas cyanuric acid up to these preferred temperatures. Thus, the reactionwill ensue substantively at temperatures such as about 350°-700° C. oreven lower, e.g., at temperatures of 400°-700° C. or 450°-600° C. ortemperatures less than 600° C. For example, temperatures on the order ofabout 400° C. will often suffice where residence times are greater thanor equal to about one second. For prior art systems, including the priorart system of JP 54-28771, by implication, substantial reduction ofNO_(x) at temperatures lower than the high temperatures typicallyrequired (e.g., less than about 900° C. for ammonia injection) does notoccur.

On the other hand, for this invention, even when an exhaust gas streamreaches a low temperature, e.g., less than 600° C., substantialreduction of NO_(x) in accordance with this invention will continue tooccur. This represents a major advantage since exhaust gas streamsinevitably cool during their passage from the point of generation. Thus,in those common situations where not all of the NO_(x) is removed duringthe time that the exhaust gas stream is at the requisite hightemperature of the prior art, this invention will continue to provideNO_(x) reduction. This invention represents a very efficient techniqueboth at temperatures greater than 600° C., e.g., 601° C. to the upperlimit discussed above and also at temperatures less than 600° C., e.g.,at temperatures from the lower limit discussed above up to 599° C.

Thus, in one Unique aspect of this invention, there is provided a methodfor at least partially decreasing the concentration of NO_(x) in anexhaust gas stream at a temperature less than 600° C., e.g., at atemperature less than or equal to 590° C., 570° C., 550° C., 500° C.,450° C., 400° C., etc.

The amount of NO_(x) reduction which might occur at such heretoforeinoperable, low temperatures will be a function of the usual parameters(see above) including the highest available temperature upstream, theresidence time upstream, the NO_(x) initial concentration, etc.

The temperature at which the HNCO or HNCO-generating solid is added tothe exhaust gas stream will also be a function of the usual parametersincluding those mentioned above. Typically, it will be desired toachieve the HNCO presence at as high a temperature as possible withinthe constraints mentioned above and the availability of suitable energy.Since a free radical mechanism is involved, a suitably high initiationtemperature will more quickly achieve a sufficiently high concentrationof radicals to cause rapid achievement of an adequately highconcentration of the active species to correspondingly quickly commenceNO_(x) reduction. Once free radical initiation ensues, the preferencefor high reaction temperatures will be lowered in accordance with theforegoing.

Another consequence of the discovery of this invention that HNCO is theactive species, i.e., that the active species is itself gaseous innature under the relevant conditions, is that certain particle sizeranges will be preferred when the HNCO is not added directly to the gasstream as a gas, i.e., is added in the form of a solid which producesHNCO, e.g., a sublimable solid such as cyanuric acid. The basicprinciple is to produce the active gaseous species as quickly andefficiently as possible. Typically, the smaller the particle-size thebetter, taking into the account the usual engineering considerations.Preferably, the average particle-size will be less than 100 microns,e.g., less than or equal to about 95, or 90, or 80, or 70, or 60, or 50,or 40, or 30, or 20, or 10, etc. Typically, the preferred particle sizeswill be in the range of 1 to less than 100 microns, most preferably inthe range of from 10 to less than 100 microns. The precise particle sizerange preferred for a given application will be a function of the usualconsiderations including system temperature, exhaust stream diameter,available residence times, the efficiency and configuration of theinjection system, etc.

As is well known, for smaller diameter streams, e.g., those typicallyencountered in vehicular systems, direct admixture of gaseous HNCO tothe exhaust gas stream is typically preferred; for larger cross-sectionsystems such as those encountered in typical smokestack exhausts, it isdifficult to achieve adequate gas-gas mixing in the available timeswhereupon injection or other addition of particles is the preferredmode. It is also possible to inject a combination of particles andgaseous HNCO, e.g., by employing an injection configuration whichprovides the possibility for preheating the particles to be injected ina chamber whereupon both gaseous HNCO and solid material are introduceddirectly into the exhaust gas system. This also enhances the initiationof the reaction for the reasons discussed above. In a relatedembodiment, it is also preferred to include in the injection material(particles, gas or a mixture thereof), HNCO generating substances whichhave a particularly favorable decomposition profile such as ammeline.

Except as indicated otherwise herein, details of the mixing of theexhaust gas stream with solid particles or directly with gaseous HNCOwill be in accordance with the usual conventional considerations asdiscussed thoroughly in the literature, e.g., in Combustion and MassTransfer, D. Bryan Spalding, Pergammon Press 1979.

As discussed above, the preferred technique is addition of gaseous HNCOdirectly to the exhaust gas stream or addition of particles havingdiameters as discussed above. In other preferred aspects, the solidmaterial can be added in the form of a solution in a solvent preferablyhot water or as a slurry in an appropriate liquid, also preferablywater, i.e., at a temperature where the solid agent is not fullydissolved. Other suitable solvents or dispersing fulids, e.g., liquidCO₂, N₂, etc., of course can also be used. These aspects provide ease ofhandling via conventional pumps.

Thus, for example, cyanuric acid can be dissolved in high temperature,high pressure water, especially in power plant environments wheresaturated water/steam is readily available, e.g., typically at 180° C.(150 psi). Solid or slurry injection can, for example, be accomplishedby the use of a lead screw connecting the exhaust stream with the powderor slurry reservoir of the solid agent such as cyanuric acid. Steaminjection is also perferred. Particle size considerations in suchslurries will be in accordance with the foregoing. Where directinjection of gaseous HNCO is employed, it will be preferred to use aheated pre-chamber to avoid occasional plugging of metering deviceswhere this may be a problem, e.g., due to polymerization of HNCO on coldsurfaces.

As can be seen from the foregoing, this invention involves the discoverythat HNCO can be used to very efficiently remove NO_(x) from exhaust gasstreams under conditions where the reaction with NO_(x) occurssubstantially solely in the gas phase with HNCO and not on the surfacesof particles of substances which can optionally be used to generate theHNCO. Accordingly, contrary to the disclosure of JP 54-28771, theconditions are to be chosen in order to facilitate the sublimation orother conversion of substantially all of such solid particles intogaseous HNCO before and/or during the reaction(s) which is effective tolower the NO_(x) concentration.

In a further feature of this invention, it has been discovered that theunderlying process is relatively insensitive to prior art interferantsincluding particulates such as fly ash and oxygen. Because of its uniquefeatures, it is especially advantageously applicable to systems whichheretofore have presented a relatively severe NO_(x) problem such assystems based on diesel or coal combustion, e.g., boilers, smokestackexhausts, etc.

Pressure is typically not a critical variable under all realisticapplications. Thus, pressures in the range from about 0.1-10 atmospheresas well as lower or higher values are employable.

The relative amounts of NO or HNCO are not critical. Typically, thesystem will be designed so that approximately stoichiometric amounts areemployed. Of course, excesses of either ingredient can be designed wheredesirable. In many applications, it will be desired to utilize veryslight, environmentally acceptable excesses of NO in order to avoidexcesses of HNCO. The latter is an acid which might recyclize tocyanuric acid at the low temperatures ensuing after the reaction has runits course. Thus, since the excess of NO can be chosen to be benignlylow in view of the great efficacy of this invention in reducing NOcontents, and since the products of the overall NO-reduction reactionsare nitrogen, carbon dioxide, water and carbon monoxide (with a minorcomponent of CO), the resultant system containing benign amounts of NOwill cause no environmental concerns. Of course, where otherwisedesirable, the system can also be run with slight excesses of HNCO.Where excesses are employed of either ingredient, these can, e.g., be inthe range of about 1.01 to about 1.1 or more on a stoichiometric basis.However, it also will often be desirable to use larger excesses of HNCOto ensure the optimum NO_(x) removed, e.g., molar equivalent excesses inthe range of 1.1-10/1, typically less than 5/1, 4/1, 3/1, 2/1, etc., orgenerally in the range of 1/1 to 5/l, etc.

In a preferred mode of operation of this invention, the NO reductionreactions will be conducted in the presence of surfaces which act as acatalyst for the free radical reactions which effect the NO reduction.The nature of the surface is not critical as long as it is catalyticallyeffective, metallic or otherwise. All those surfaces well known tocatalyze related free radical reactions will be employable, metallicsurfaces, oxides, etc. For metallic systems, preferably, the metalcomponent will be iron which will typically be provided by the steel,stainless steel, or other iron-based surfaces utilized in plants,vehicles, factories, etc., and especially utilized in the conduitscontaining effluent gas streams, e.g., mufflers, smokestacks, etc. Othertypical metals include the usual transition metals, e.g., the noblemetals, including platinum, palladium, rhodium, silver, gold, etc. aswell as nickel, cobalt, chromium, manganese, vanadium, titanium, etc. Ina further preferred embodiment, the reaction will be conducted in achamber containing particles of such catalytic surfaces, e.g., pellets,beads, granules, etc. The particle sizes and distributions are notcritical. As usual-, the greater the surface area, the more efficientthis effect will be. Where catalytic surfaces are utilized, residencetimes can be shorter and temperatures can be lower under otherwiseidentical conditions. Without wishing to be bound by theory, it is feltthat the catalytic effect is primarily exerted in initiating thegeneration of free radicals triggering chain reactions necessary for theNO reduction.

Other components may also be present in the NO-containing stream withoutadversely impacting this invention. For example, where NO₂ is involved,it also will be removed by this invention. However, under the normalconditions where NO is a problem, NO₂ often is not a problem. The amountof NO in the effluent gas stream also is not critical. Typically, theamounts will be 1 ppm or more, e.g., 1-10,000 ppm or 10-5,000 ppm,typically 100-1,000 ppm, etc. By routine, judicious selection ofreaction conditions as described above, the amount of NO after admixturewith HNCO can be reduced to any desired low level, including 0 ppmwithin limits of detection. Greater reductions in NO contents in a givensystem can be achieved by utilizing longer residence times and highertemperatures.

FIG. 1 illustrates one embodiment of a system of this invention. Theoverall "device" 1 simply comprises means such as chamber 2 for holdingthe sublimable compound; means for heating the latter to its sublimationtemperature, e.g., in FIG. 1 the means simply being the input gas stream3 which is at an elevated temperature; means for contacting theresultant HNCO with the input stream, which here simply comprises theadjoining conduits whereby the input stream heats the cyanuric acid andthe resultant HNCO is instantaneously mixed with the input stream; andmeans for conducting the reaction, here illustrated by furnace 4. Manyother equivalents will be very clear to skilled workers. For example,one or both of the storage chamber and the furnace can be inductively,conductively, radiatively, etc., heated using external sources otherthan the input stream itself. One or both of storage chamber and furnaceregion can be located anywhere along the oath of the effluent stream,e.g., they can be located right at the head of an engine or the exhaustoutlet of a furnace or plant. As discussed above, it is even possiblefor the storage means to be located upstream of the chamber whichproduces the effluent stream where this is practical. Conventional heatexchange means can also be incorporated into the system whereverdesirable. In FIG. 1, the heat exchange means is the input gas itself.

Without wishing to be bound by theory, the following is a proposedmechanism for the NO reduction system: ##STR4##

As can be seen, free radicals ace generated which cause chain reactionsto ensue. This explains both the speed and high efficiency of the systemin removing NO from the gas stream. The reaction mechanism is highlysurprising since the weakest bond in the HNCO molecule has a strength ofabout 85 kcal whereupon it would have been expected that a much highertemperature than those in the range of 400°-800° C. would be necessaryfor a significant effect based on decomposition of HNCO.

This mode of action also serves to further clarify the distinctionbetween this invention and the more conventional chemistry known forHNCO, e.g., that is described in Back et al., supra. In the latter, noelevated temperatures were used; only a purely photolytic decompositionof HNCO was effected. In addition, the lowering of NO content mentionedin this reference related only to relatively high pressures of NO in theseveral torr range.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description; utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius and unless otherwise indicated, allparts and percentages are by volume.

The entire text of all applications, patents and publications, if any,cited above and below are hereby incorporated by reference.

EXAMPLE 1

A 7.2 horsepower Onan diesel engine was utilized for the experiment. Itsexhaust had a flow-rate of 100 l/m. A 2 l/m sample was introduced into acyanuric acid sublimation chamber. The latter contained 50 g of cyanuricacid and the sublimation occurred at 350° C. Thereafter, the mixture ofHNCO and exhaust gas was passed through a furnace region packed with abed of steel ball bearings. The temperature in the furnace region wasmaintained at a temperature equal to or greater than 450° C. utilizing aconventional heater. The effluent from the furnace region was passedinto a NO_(x) analyzer. The residence time in the furnace was about 1second.

The exhaust gas from the diesel engine included the usual soot, water,oxygen and CO₂. Its 500 ppm NO content was reduced to less than 1 ppm(i.e., to the sensitivity level of the NO_(x) analyzer). The load on theengine varied from 0.23 to 0.8 with no effect observed on the process.

EXAMPLE 2

Under the conditions of Example 1, 5 pounds of cyanuric acid (2.27 kg)is loaded into the holding chamber. This provides enough activeingredient (53 moles of HNCO) to remove approximately 50 moles of NO. Ata typical NO concentration in a vehicle exhaust of 500 ppm, 2.5×10⁷liters of gas can be scrubbed. This is sufficient to remove NO from theexhaust gas of automobiles for a driving range of approximately 1,500miles.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A method of reducing the NO content of a gasstream containing NO comprising contacting the gas stream with an amountof cyanuric acid under conditions effective for the thermaldecomposition thereof, said amount and the conditions of the gas streambeing effective for resultant lowering of the NO content of the gasstream, said cyanuric acid being particulate and having a particle sizeof less than 90 μm.
 2. A method of claim 1, wherein said particle sizeis from 1 up to less than 90 μm.
 3. A method of claim 1, wherein saidparticle size is less than 70 μm.
 4. A method of claim 1, wherein saidparticle size is less than 50 μm.
 5. The method of claim 1, wherein saidNO reduction is effected by chemical reactions conducted at temperaturesin the range of 350°-850° C.
 6. A method of claim 1, wherein said NOreduction is effected by chemical reactions conducted at temperatures inthe range of 350°-800° C.
 7. A method of claim 1, wherein said NOreduction is effected by chemical reactions conducted at temperatures inthe range of 350°-750° C.
 8. A method of claim 5, wherein the particlesize of cyanuric acid is 10-90 μm.
 9. A method of claim 5, wherein saidgas stream is exhaust from a diesel engine.
 10. A method of claim 7,wherein said gas stream is exhaust from a diesel engine.
 11. A method ofclaim 5, wherein said particle size is less than 70 μm.
 12. A method ofclaim 5, wherein said particle size is less than 50 μm.
 13. A method ofreducing the NO content of a gas stream containing NO comprisingcontacting the gas stream with an amount of cyanuric acid particles of asize less than 90 μm under conditions effective for their thermaldecomposition, said amount being effective for resultant lowering of theNO content of the gas stream.